Frequency-translation devices (FTDs) such as mixers, converters, and tuners are important components in many radio-frequency (RF) and microwave communication systems. For instance, FTDs are important components of wireless telephones, satellite communication systems, and wireless internet devices, to name but a few.
As communication systems adopt more advanced types of modulation, FTD designs tend to become increasingly complex, and their specifications more demanding. Accordingly, to ensure proper performance, test equipment should be designed to accurately characterize the behavior of these increasingly complex FTDs. In general, an FTD should have substantially linear phase shift and flat group delay across its modulation bandwidths.
FTDs can be difficult to characterize because their input and output frequencies differ. Accordingly, FTDs rely upon different measurement techniques than those used to characterize linear devices such as filters. In addition, many FTDs, such as multi-channel frequency converters, are relied upon to be characterized using several different local oscillator (LO) frequencies and across many different input and output frequency ranges.
A number of techniques have been proposed for characterizing phase and group delay response characteristics of contemporary FTDs. In each of these techniques, an FTD is characterized by comparison with a reference device such as a calibration mixer. Accordingly, these conventional techniques do not provide for direct determination of FTD phase and group delay response characteristics.
In one conventional example, a test system determines the phase response of an FTD by measuring the relative phase responses of three sets of mixer pairs and then solving three resulting equations with three unknowns to determine the phase response of each mixer. This technique, however, is susceptible to mismatch errors introduced by interaction between the various mixers and an intermediate frequency (IF) filter used to select a desired mixing product. This method also relies upon that one of the mixers be reciprocal. Additionally, it does not scale well to frequency converters with multiple conversion stages, because it relies on a reconversion mixer driven with a single LO.
In another conventional example, a test system is first calibrated by placing a calibration mixer with known characteristics into a test channel. Then, an FTD is placed in the test channel and its performance is measured relative to a calibrated reference device in a reference channel. In this example, FTD measurements are limited to a frequency range of the calibration mixer. In addition, the calibration mixer is reciprocal. Because of these limitations, this technique cannot be readily applied to complex converter designs.
In still other examples, two-tone stimuli or modulated-signals are applied to an FTD to determine its phase response characteristics. These examples also rely upon a calibration mixer and are therefore difficult to apply to complex converter designs.
Because of the above limitations, conventional test systems for FTDs can be cumbersome and expensive to use. What is needed, therefore, are a network analysis system and a method of calibrating a network analysis system that overcomes at least the shortcoming described above.